Currently, Reference Signal-Signal to Noise and Interference Ratio (RS-SINR) is a User Equipment (UE) radio measurement agreed in 3rd Generation Partnership Project (3GPP) for multi-carrier load distribution. The measurement period for RS-SINR is the same as the measurement period for the legacy Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ), i.e., 200 ms. The RS-SINR measurement definition from 3GPP TS 36.214 v.13.1.0 is as the linear average over the power contribution (in Watts) of the resource elements carrying cell-specific reference signals divided by the linear average of the noise and interference power contribution (in Watts) over the resource elements carrying cell-specific reference signals within the considered measurement frequency bandwidth.
Typically, measurement accuracy increases in better interference conditions, e.g. improving signal the ratio of received energy to total interference (Es/Iot) is generally expected to improve measurement accuracy. However, the opposite has been observed analytically and based on simulations. In particular, at a high Es/Iot, the RS-SINR measurement accuracy may degrade (e.g., by 1 dB at Es/Iot=16 dB) and one of the reasons is a static error due to a frequency shift, also referred to herein as frequency error, seen at the receiver.
The frequency error is the difference between the UE carrier frequency (F1) on which the UE receives a signal from a network node or another UE and the carrier frequency (F1′) on which the network node or another UE transmits the signal. The frequency error may be determined between the center frequencies, F1 and F1′. Ideally frequency error between F1 and F1′ should be zero or negligible. But due to UE impairments the frequency error is non-negligible.
The frequency error may be different on an uplink (UL) frequency and a downlink (DL) frequency, e.g. in FDD, HD-FDD schemes, among others. For example, in UL the frequency error is the difference between the UE carrier frequency (F2) on which the UE transmits a signal to a network node or another UE and the carrier frequency (F2′) on which the network node or the another UE receives the signal.
Frequency error is also referred to as frequency offset, frequency deviation etc. It can be expressed in parts-per-million (PPM), parts-per-billion (PPB), Hz, KHz etc. The frequency error also depends on one or more of: UE speed and the value of the frequency. Typically frequency error increases with an increase in frequency, e.g. ±50 Hz error at 1 GHz and ±100 Hz error at 2 GHz). As an example the UE modulated carrier frequency is accurate to within ±0.1 PPM observed over a period of one time slot (0.5 ms) compared to the carrier frequency received from the network node e.g. E-UTRA Node B.
FIGS. 1 and 2 illustrate degradation of RS-SINR measurement accuracy with increasing Signal to Noise Ratio (SNR) for different levels of frequency error or shift. FIG. 1 illustrates a graph of a simulated measurement of RS-SINR on the y axis versus an actual SNR of the reference signal on the x axis when a frequency error is 20 Hz. FIG. 2 illustrates a graph of a simulated measurement of RS-SINR on the y axis versus an actual SNR of the reference signal on the x axis when a frequency error is 50 Hz. As illustrated by these graphs, an increase in frequency error exacerbates the RS-SINR measurement inaccuracy. The frequency error may be caused by different reasons or conditions, e.g., transmitter imperfections, receiver imperfections, UE clock inaccuracy, high carrier frequency (e.g., above a threshold), etc.
Time offset error is the difference between the time instance the UE receives a signal from a network node or another UE and the time instance the network node or the other UE transmits the signal. This includes timing error due to UE impairments, propagation delay between the UE and the network node or the other UE, and also multipath propagation delay. The time offset is also referred to as timing error etc. It is typically expressed in microseconds or, alternatively, in basic time units of Ts seconds, where Ts is specified in the relevant 3GPP standard specifications to be 32.55 ns.